Here’s what the team had to say about hitting the 400 million user mark:

Our community has evolved to be even more global, with more than 75 percent living outside of the US. To all the new Instagrammers: welcome! Among the last 100 million to join, more than half live in Europe and Asia. The countries that added the most Instagrammers include Brazil, Japan and Indonesia.

Yes, it’s no mistake that the team wanted to let all of us know that it’s growing internationally, the key to grabbing its “next 400M.” It’d be sure nice to know what the splits are on iOS vs. Android, though.

For anyone who was worried that ugly Instagram ads would scare users away in droves, that’s not the case. This is good news for advertisers, who are starting to fill up the network with both photos and video commercials.

Our own Josh Constine pretty much nailed why Instagram continues to blow up when the group announced the 300 million mark last December:

…Instagram can rest easy knowing it captured lightning in a bottle. It’s the archetypal mobile app. There were plenty of other ways to share photos before Instagram. There were plenty of other ways to filter photos too. But Instagram took something people already liked to do and made it so simple and delightful they fell in love.

And yup, Instagram sports a bigger user base than Twitter, which last reported 316 million users. Seems like Instagram has cracked the code of finding users in every nook and cranny of the Internet all over the world. With Facebook’s help, of course.

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Crunchbase

OverviewInstagram is a free photo sharing application that enables its users to take photos, apply filters, and share them on social networks such as Facebook, Twitter, Foursquare, Tumblr, Flickr, and Posterous. It allows its users to capture and customize their photos and videos with several custom-built filter effects.
Developed by Kevin Systrom and Mike Krieger, Instagram is compatible with all iOS …